A "slug" is a small creature that looks like a snail without a shell. It moves very slowly and leaves a slimy trail behind. When people use the word "slug" in other contexts, it can mean a bullet or a slow and lazy person. Full definition
At any rate, the core idea and execution of Metal Slug as a defense game is right on the money. (toucharcade.com)
Speaking of the keyless entry system, the Maserati transponder key is a heavy slug of metal that feels nice and substantial in the hand, but is probably a bit bulkier than strictly necessary. (cnet.com)
One afternoon in the late 1980s, a young marine biologist named Drew Harvell unearthed dozens of sea slugs in an upstate New York warehouse. (discovermagazine.com)